The present invention generally relates to a wafer boat for holding wafers in a vertical furnace during an oxidation process and more particularly, relates to a wafer boat that has improved wafer holding capability during an oxidation process in a vertical furnace such that the necessity of loading and unloading dummy wafers from the boat for each oxidation process can be avoided.
In connection with processes used to manufacture semiconductor devices, such as integrated circuits, numerous process steps are carried out in a controlled environment at elevated temperatures. Such processes includes oxidation, diffusion, chemical vapor deposition and annealing. In order to realize elevated processing temperatures, semiconductor wafers are processed in an evacuated chamber, typically in a form of a quartz tube which is housed within a semiconductor furnace.
The most common type of semiconductor furnace is of the xe2x80x9chot wallxe2x80x9d electric type which facilitates batch processing of semiconductor wafers. Furthermore, hot wall electric furnaces exhibit excellent temperature stability and precise temperature control. Modern hot wall diffusion furnaces are capable of controlling temperatures over the range of 300xc2x0 C. -1200xc2x0 C. to an accuracy of xc2x10.50xc2x0 C. Hot wall furnaces were initially designed as horizontal diffusion furnaces, however, more recently, vertical furnaces have gained favor because they present a number of advantages over their horizontal predecessors. These advantages include: elimination of cantilever or soft-landing since the wafers are held in a quartz boat which does not touch the process tube walls; wafers can be loaded and unloaded automatically; and, the clean room footprint of the system is somewhat smaller than that of the conventional horizontal configuration. Vertical semiconductor furnaces of the type mentioned above employ a quartz tube which typically has a polysilicon coating when used for a deposition or annealing process. The polysilicon deposition reduces the power loss due to quartz reflection or radiation, and reduces the degradation of a boat occasioned by wet etching. Because semiconductor furnaces are subjected to high rates of usage and their components are exposed to harsh operating environments, periodic maintenance must be performed on various furnace components, including the quartz tube assembly.
A typical vertical furnace 10 and a typical wafer boat 20 are shown in FIGS. 1A and 1B. The wafer boat 20 is fabricated of a quartz material in order to survive the extreme high process temperature. The vertical oxidation furnace 10 is also constructed of quartz material to sustain the high process temperature of up to 1200xc2x0 C. As shown in FIG. 1A, the vertical quartz furnace 10 is heated by an electrical resistance coil 12 which is capable of providing a temperature control accuracy of 0.5xc2x0 C. in the operating temperature range of 300xc2x0 C. xcx9c1200xc2x0 C. The wafer boat 20, shown in FIG. 1B is frequently constructed of a top plate 14, a bottom plate 16 connected thereinbetween by vertical support posts 18 which also provides cavities for housing the wafers therein. For instance, as shown in FIG. 1B, a plurality of wafers 22 are positioned in cavities 24 which are formed by horizontal ridges 26. Since a typical eight inch wafer has a thickness of about 0.6 mm, a minimum height of cavity 24 should be about 3.5 mm in order to avoid the top surface 28 of wafer 22 being scratched by the robot blade used in loading and unloading the wafers.
In a conventional wafer oxidation process, the wafer boat 20 holds a total of 172 wafers. However, it is customary to run six batches of wafers stored in wafer cassettes of 24 wafers each, amounts to a total of 144 wafers. It has been found that at the top and at the bottom of the vertical furnace, the temperature achieved for oxidation with the boat loaded in the furnace cannot be accurately controlled. It is therefore customary to load a number of dummy wafers at the top and at the bottom of the wafer boat in order to achieve accurate temperature control on the production wafers that are loaded in the center portion of the wafer boat. For instance, it is customary to load approximately nine dummy wafers into the top portion of the wafer boat and approximately fifteen dummy wafers into the bottom portion of the wafer boat. A more effective heating, i.e.,and thus more effective oxidation, can be achieved by the use of dummy wafers in the wafer boat.
While the loading of dummy wafers in the wafer boat improves the overall oxidation process for the production wafers, it does create other processing problems such as the additional time required for loading and unloading the dummy wafers. In order to save time for such loading/unloading, it is preferred to only load/unload the production wafers before/after each oxidation process while leaving the dummy wafers in their cavities. While vibrations are frequently caused in the loading/unloading of a wafer boat into the furnace, i.e., by moving on a boat elevator, it is not unusual that the dummy wafers, after repeated cycles of loading/unloading, are shifted out of their cavity and thus presenting a serious problem during the upward/downward movement of the wafer boat in the furnace. The dummy wafers may either suffer breakage or cause serious damages to the furnace or the wafer boat.
In accordance with the present invention, a wafer boat that has improved wafer holding capability and a method for loading dummy wafers into a wafer boat without wafer sliding-out problems are provided.
In a preferred embodiment, a wafer boat for holding wafers during processing in a vertical furnace can be provided which includes a top plate and a bottom plate connected together by four spaced-apart, vertically extending support posts, the four spaced-apart support posts are arranged in a circumference and consist of two outer support posts positioned juxtaposed to an inlet of the wafer boat and two inner support posts positioned away from the inlet of the wafer boat, the four spaced-apart support posts are further arranged in a circumference of the wafer boat; a plurality of cavities horizontally disposed each for supporting a wafer therein, each of the plurality of cavities being formed by a horizontal notch in one of the two inner support posts facing a center of the wafer boat, and a horizontal ridge integrally formed with one of the two outer support posts extending radially inwardly toward a center of the wafer boat and circumferentially outwardly toward the inlet of the wafer boat; and a raised portion extending upwardly from a top surface of each of the horizontal ridge adjacent to the inlet of the wafer boat for preventing a wafer from sliding out of one of the plurality of cavities.
In the wafer boat for holding wafers for processing in a vertical furnace, the plurality of cavities in the wafer boat is sufficient for holding 172 wafers, or sufficient for holding both production wafers and dummy wafers. The top plate, the bottom plate, the four support posts and the horizontal ridges are fabricated of quartz. The raised portion of the top surface of each of the horizontal ridges has a thickness of at least 0.2 mm, or a thickness between about 0.2 mm and about 2 mm. Each of the plurality of cavities horizontally disposed has a height between about 3 mm and about 5 mm, or a height of about 3.5 mm. The raised portion on the horizontal ridge has a thickness of about 0.2 mm.
The present invention is further directed to a method for loading dummy wafers into a wafer boat without sliding-out problem which can be carried out by the operating steps of providing a wafer boat that has a plurality of cavities horizontally disposed therein each adapted for supporting a wafer; the wafer boat includes a top plate and a bottom plate connected together by four spaced-apart, vertically extending support posts, the support posts are arranged in a circumference and consist of two outer support posts positioned juxtaposed to an inlet of the wafer boat and two inner support posts positioned away from the inlet of the wafer boat; each of the plurality of cavities is formed by an horizontal notch in one of the two inner support posts facing a center of the wafer boat, and a horizontal ridge integrally formed with one of the two outer support posts extending radially inwardly toward the center of the wafer boat and circumferentially outwardly toward the inlet of the wafer boat; and a raised portion extending upwardly from a top surface of each of the horizontal ridge adjacent to the inlet of the wafer boat for preventing a dummy wafer from sliding out of one of the plurality of cavities; extending a dummy wafer supported on a robot blade into one of the plurality of cavities; lowering the robot blade to position the dummy wafer in one of the plurality of cavities with an edge of the dummy wafer positioned radially inside the raised portion on the top surface of the horizontal ridge; and withdrawing the robot blade from one of the plurality of cavities.
The method for loading dummy wafers into a wafer boat without sliding-out problem may further include the step of raising the wafer boat into a furnace or conducting an oxidation reaction and lowering the wafer boat to remove oxidized production wafers without having to remove or reload the dummy wafers. The method may further include the step of providing the raised portion on the horizontal ridge in a thickness between about 0.2 mm and about 2 mm. The method may further include the step of providing the plurality of cavities horizontally disposed each has a height between about 3 mm and about 5 mm. The method may further include the step of providing the wafer boat in a quartz material, or the step of loading at least 5 dummy wafers into each of top and bottom cavities in the wafer boat. The method may further include the steps of loading 9 dummy wafers into a top portion of the wafer boat and loading 15 dummy wafers into a bottom portion of the wafer boat. The method may further include the step of loading at least 144 production wafers into the wafer boat prior to conducting an oxidation reaction.
It is therefore an object of the present invention to provide a wafer boat for holding dummy wafers in repeated oxidation processes without the drawbacks or shortcomings of the conventional wafer boats.
It is another object of the present invention to provide a wafer boat that has improved wafer holding capability such that dummy wafers are not shifted out of their cavities after repeated loading/unloading operations.
It is a further object of the present invention to provide a wafer boat that has improved wafer holding capability wherein a raised portion is provided on horizontal ridges forming the cavities for preventing the wafer from sliding out.
It is another further object of the present invention to provide a wafer boat for securely holding wafers during processing in a vertical furnace that is capable of holding wafers securely in their cavities even after repeated movements into and out of a furnace.
It is still another object of the present invention to provide a wafer boat for securely holding wafers during processing in a vertical furnace wherein wafer cavities are formed by horizontal notches in support posts and horizontal ridges formed with support posts.
It is yet another object of the present invention to provide a wafer boat that has improved wafer holding capability that is capable of sustaining processing temperatures as high as 1200xc2x0 C.
It is still another further object of the present invention to provide a wafer boat that has improved wafer holding capability which is fabricated of a quartz material.
It is yet another further object of the present invention to provide a wafer boat that has improved wafer holding capability that can hold at least 144 production wafers and 24 dummy wafers therein.